1. Field of the Invention
The present invention relates to a wireless charging system and, more particularly, to a wireless charging system with foreign metal object detection capability and a foreign metal detection method for the system.
2. Description of the Related Art
Wireless charging by far mainly relies on technique associated with near-field magnetic induction, which is characterized by a wireless charger applied in a preset charging area to transfer power to power-consuming equipment. The power-consuming equipment charges a battery thereof and supplies itself for operation with the received power. The concept behind the wireless power transfer resides in that the wireless charger is built in with a coil with AC (Alternating Current) current passing through the coil in generation an AC magnetic field and electrical energy is transferred from the AC magnetic field to a coil in the power-consuming equipment and stored in a battery of the power-consuming equipment. Wireless charging certainly increases operational convenience of power-consuming equipment in charging. However, when any foreign metal object enters the charging area, there is a high chance of damage to the power-consuming equipment and a great danger arising from high temperature generated by the foreign metal object during wireless charging.
Currently, foreign object detection technique for wireless charging can be classified into two types, namely, technique using external sensor, such as coil, ultrasonic sensor, infrared sensor and the like, and technique using power transmission coil. The technique using external sensor detects foreign object on a surface of the antenna at the transmitter side while the external sensor affects the power transmission performance of the wireless charging system due to mutual interaction, such that such technique is only appropriate to detection prior to charging. The technique using power transmission coil determines existence of foreign object depending on variations of measured parameters. The measured parameters include transmission power, transmission efficiency, S-parameter, quality factor and the like. However, those parameters are all subject to unmeasurable variations arising from changes in transmission distance, and are not accurate for determining if foreign object actually exists.
As disclosed in Taiwan Patent Publication No. 201415064, entitled “Systems, methods and apparatus for detecting foreign objects”, an apparatus for detecting a presence of an object in a magnetic field is provided. The apparatus includes a power circuit configured to generate the magnetic field and transfer power wirelessly at a level sufficient to supply power to or charge a load via the magnetic field, and the magnetic field causes a vibration of the object. The apparatus further includes a detection circuit configured to transmit signals and detect if the object exists based on a reflection of the transmitted signals and a difference between a measured characteristic and a corresponding characteristic of a frequency of the vibration of the object caused by the magnetic field.
As can be seen from the foregoing conventional technique, should any foreign metal object enter a charging area, there is a high likelihood that the high temperature generated by the foreign metal object during charging will damage the equipment in charging and cause danger to surrounding people. However, in the conventional techniques the external sensor could readily affect power transmission performance, and the power transmission coil for detecting foreign objects fails to accurately detect a presence of foreign objects when transmission distance varies. Although the conventional techniques detect the presence of foreign objects through variation of measured resonant frequency, the resultant element structure tends to be complicated and thus results in the production cost increase and the conventional techniques fails to perform accurate foreign object detection.